Method of performing hydrolysis and alkalic digestion of cellulosic fiber material with prevention of lignin precipitation



Nov. 26, 1968 E. AKE BACKLUND METHOD OF PERFORMING HYDROLYSIS AND ALKALIC DIGESTION OF CELLULOSIC FIBER MATERIAL WITH PREVENTION OF LIGNIN PRECIPITATION Filed Jan. 25, 1965 59 f, r 57 2 5 i 1' L ZjLM/J 55 as '(Q iiif l/vvrae United States Patent 3,413,189 METHoD 0F PERFORMING HYDROLYSIS AND ALKALrc mGEsTIoN 0F CELLULOSIC FIBER MATERIAL wirn PREVENTION on LIGNIN PRECIPITATION Ernst Ake Baclclund, Karlstad, Sweden, assignor to Alrtiebolaget Karnyr, Karlstad, Sweden, :1 company of Sweden Filed Jan. 25, 1965, Ser. No. 427,814 Claims priority, application Sweden, Jan. 29, 1964, 1,660/ 64 Claims. (Cl. 16229) ABSTRACT OF THE DISCLOSURE The present invention relates to a method of performing hydrolysis and a subsequent alkalic digestion of cellulosic fiber material in one and the same vessel through which the material is continuously advanced, hydrolysate and spent liquor being separated from the fiber material and discharged together. The hydrolysate i acidic and therefore, under certain conditions of operation, particularly if said conditions vary, the pH-value of the mixture of hydrolysate and spent liquor may decrease in con- 5 nection with its discharge, at least temporarily, to such a low value that there is a risk of alkali lignin being precipitated out of the spent liquor and deposited in the discharge conduit so that the latter is choked.

According to the present invention, said risk is eliminated and an undisturbed operation is obtained by the feature that a portion of the digesting liquor is supplied in the vicinity of the spot where the hydrolysate and the spent liquor are mixed. Then the pH-value in the dis charge conduit will be sufficicntly high to prevent deposits therein. Preferably said liquor portion is chosen suflicient- 1y great to neutralize the acidic or acid-forming substances of the hydrolysate.

Above all the invention will be applied where the delignifying digestion is performed with the liquor flowing 50 counter-currently to the fiber material towards a discharge sieve inserted in the digester shell, through which sieve also hydrolysate is let off. Then the invention is applied in such a manner that said liquor portion is mixed into the hydrolysate and is drawn oil through said sieve together with the hydrolysate. Thereby the hydrolysate will not be in a position to cause precipitation of alkali lignin out of the spent liquor, but the discharge sieve will be maintained clean.

The invention also relates to an upright cylindrical Vessel for performing prehydrolysis and alkalic digestion of fiber material axially fed therethrough, which vessel is provided with a sieve girdle inserted in the shell thereof and adapted for drawing-off prehydrolysate as well as Patented Nov. 26, 1968 spent digesting liquor. In order to be able to perform the above-mentioned method in such a vessel, the same is characterised in that from a conduit for supplying the white liquor required for the digestion there is branchedotf a conduit which opens out into the vessel approximately at the level of said sieve girdle and preferably centrally of the cross-section of the vessel, so that the white liquor supplied through said last-mentioned conduit moves in a substantially horizontal and preferably radial direction towards the sieve girdle.

The invention will be more closely described herein below with reference to the accompanying drawing.

In the drawing, 11 designates an upright cylindrical vessel of approximately uniform cross-section and a length of the order of 10 times the diameter. At its upper end the vessel is provided with known device for charging wood chips or some other suitable comminuted fiber material from which cellulose should be produced. At its lower end the vessel is provided with known devices for discharging the fiber material. The fiber material, the path of which through the vessel is designated by thick arrows 13, moves as a compact column axially downwards through the vessel, and during said movement it is first hydrolized, then delignified and finally washed as more closely described below.

Approximately at the middle of the vessel a sieve girdle 15 is inserted into the vessel shell. Prehydrolysis of the fiber material takes place in the part of the digester located above said sieve girdle. Therefore, either water or a diluted acid is fed to the top of the vessel through the conduit 17. Said liquid is drawn oil through a sieve girdle 19 inserted in the digester shell and is circulated by means of a pump 21 in a loop comprising a heating device 23 where heating takes place by means of steam supplied through the conduit 25, and which circulation loop has an orifice located centrally in the digester at approximately the same level as the sieve girdle 19. Due to said circulation the fiber material is heated to the temperature required for hydrolysation. The hydrolysate flows concurrently to the fiber material, as indicated by the arrows 25, and is discharged through the sieve girdle 15 and the conduit 27 connected thereto.

In the zone of the vessel located below the sieve girdle 15 a delignifying digestion by means of sulphate liquor or some other alkalic liquor is performed, Said liquor is supplied through a conduit 29 connected to a loop comprising a sieve girdle 31 inserted in the shell of the digester, a pump 33, a heating device 35 and a return conduit 37 which has an orifice located centrally in the digester approximately at the level of the sieve girdle 31. The circulating liquor is heated by steam supplied through the conduit 39 and after being distributed over the cross-section of the fiber material column the liquor passes upwardly through the same, as indicated by the arrows 41, i.e. counter-currently to the motion of the fiber material. The temperature and concentration of the white liquor as well as the velocities of the fiber material and the liquor are adapted such that the fiber material is completely delignified when passing the sieve girdle 31 and such that the greater part of the alkalic content of the liquor, while dissolving the lignin of the fiber material, is spent on its way up towards the sieve girdle 15. The black liquor thus obtained encounters the hydrolysate at the level of the sieve girdle 15 and departs to- 3 gether therewith through the conduit 27 connected to the sieve girdle.

In the zone below the sieve girdle 31 the digester is used for washing liquor out of the pulp by means of pure water or white water obtained from a subsequent washing filter. The -wash Water is supplied through the conduit 43 to the lower end of the digester. Somewhat higher up the same is drawn off through a sieve girdle 45 and is pumped by means of a pump 47 through a heating device 49 in which it is heated by steam supplied through the conduit 51, and then back to the digester into which the Wash water is supplied centrally and approximately at the same level as the sieve girdle 45. The wash water is forced upwards counter-currently to the fiber material, as indicated by the arrows 53, and displaces the digesting liquor which is forced out through the sieve girdle 15.

According to the invention the white liquor supplied through the conduit 29 is divided up into two portions, out of which the main portion, e.g. approximately two thirds, is introduced into the circulation loop 31, 33, 35, 37 and is driven counter-currently to the motion of the chips column up through the same to the sieve 15, whereas the remaining portion is branched oii into a conduit 55 which opens out centrally in the digester approximately at the level of the sieve girdle "15. The latter white liquor portion is spread from the orifice of the conduit 55 radially outwards towards the sieve girdle 15, thereby being mixed with the hydrolysate flowing concurrently to the chips column and neutralizing said hydrolysate, at least partially, so that the same, when it is subsequently mixed also with the black liquor coming from below and departs together therewith through the sieve girdle 15 and the connected conduit 27,, does not cause deposits of alkali lignin contained in the black liquor. The added white liquor brings about that the pH-value is not lowered too much by the acidic hydrolysate but is held at a sufficiently high value, e.g. pH=l or 11.

The orifice of the conduit 55 may be located at a level in the di gester somewhat above the level of the sieve girdle 15. Then the white liquid portion supplied through the conduit 55 will move a short distance concurrently to the fiber material before it is diverted together with the hydrolysate towards the sieve girdle in the shell of the digester.

A pump 57 is inserted into a circulation conduit 59 connected to the sieve girdle .15 and also to the conduit 55 opening out at the centre of the digester. By means of said pump a heavy radial flow is maintained, whereby thewhite liquor is thoroughly mixed with the hydrolysate, thus equalizing the pH-value of the mixture.

The conduit 55 may be provided with a plurality of outlets distributed over the cross-section of the vessel. Thus, in addition to the central outlet the conduit 55 may also have a series of oulets located close to the shell of the vessel immediately above the sieve girdle 15. A part of the white liquor supplied to said peripheral outlets will flow along the inner surface of vessel and safeguard that the pH-value is maintained sufiiciently high at the uppermost part of the sieve girdle, through which pure hydrolysate else might pass and cause deposits.

The above described embodiment is merely an example, the details of which may be modified also in other respects within the scope of the invention as defined by the following claims. Thus the white liquor portion used for raising the pH-value at the discharge spot can also be supplied at such a place that it moves counter-currently to the chips column. The invention will also be applicable in connection with prehydrolysis followed by one or more stages of concurrent delignifying digestion.

I claim:

1. Method of performing hydrolysis and a subsequent alkalic digestion of cellulosic fiber material in one and the same vessel wherein the material is continuously fed, in which method hydrolysate and'spent digesting liquor are separated from the fiber material and discharged 4 together, characterised in that a portion of the fresh digesting liquor is supplied in the vicinity of the spot where hydrolysate and spent digesting liquor are mixed.

2. Method as claimed in claim 1, in which said digesting liquor portion is at least sufiicient to neutralize the acidic and acid-forming substances of the hydrolysate.

3. Method as claimed in claim 1, in which said digesting liquor portion is so great that the pH-value of the mixture separated from the fiber material is raised to approximately 10 to 11.

4. Method as claimed in claim 1, in which the digestion is performed with liquor flowing counter-currently to the fiber material towards a sieve inserted into the digester shell, through which sieve also hydrolysate is dischanged, wherein said fresh digesting liquor portion is supplied in such a manner that it is mixed with the hydrolysate and moves with the same towards the sieve.

5. In a method for performing hydrolysis and subsequent alkali digestion of cellulose fiber material in one and the same vessel wherein cellulosic fiber material is supplied continuously to one end of an upright digester, moved concurrently with a hydrolyzing material through a prehydrolysis zone, separated from the hydrolysate at a sieve girdle, moved countercurrently of a digesting liquor which is also separated and discharged with said hydrolysate at said sieve girdle, moved through a washing zone countercurrently with wash water, and discharged from the other end of said digester, the improvement which comprises supplying a portion of the fresh digesting liquor to the area where hydrolysate and spent digesting liquor are mixed, whereby the pH of the mixture of hydrolysate and spent digesting liquor is sufiiciently high to prevent substantial precipitation of alkali lignin therefrom.

6. The method as claimed in claim 5, wherein the resultant mixture of said fresh digesting liquor, said hydrolysate, and said spent digesting liquor is sufliciently alkaline to prevent alkali lignin from being precipitated out of said mixture.

7. The method as claimed in claim 5 wherein said fresh digesting liquor portion is so great that the pH of the mixture separated from the fiber material is approximately 10 to 11.

8. An upright cylindrical vessel, means for continuously charging a comminuted fiber material into one end of said vessel and moving the same axially through said vessel, means for supplying a hydrolizing agent to the fiber material charged into said vessel, a conduit for supplying an alkalic digesting liquor to the vessel for delignifying the fiber material therein, said conduit having an outlet into the vessel distant from the fiber material charging end therof, a sieve girdle inserted in the shell of the vessel intermediate and distant [from the fiber material charging end and said liquor conduit outlet, said sieve girdle being adapted for separating hydrolysate mixed with spent digesting liquor from the fiber material, a conduit connected to said sieve girdle for discharging said mixture from the vessel, a branch conduit connected to said digesting liquor supply conduit and having an outlet into the vessel located approximately at the level of said sieve girdle, whereby fresh alkalic liquor is added to the mixture of hydrolysate and spent digesting liquor in order to raise the pH-value thereof and prevent clogging of said sieve girdle and the conduit connected thereto.

9. An upright prehydrolysis and alkali digestion vessel, means for continuously axially feeding a cellulosic fiber material and a hydrolysis agent to one end of said vessel, a conduit for continuously charging an alkali digestion liquor to said vessel at a location distant from the fiber charging end therof, a sieve girdle inserted axially inside said vessel between said fiber feeding means and said digestion liquor conduit, a branch conduit from said digestion liquor conduit which opens into said vessel ap proximately at the level of said sieve girdle and prefer- 5 6 ably centrally of the cross-section of said vessel, a hy- References Cited drolysate and spent digestion liquor discharge conduit UNITED STATES PATENTS from said sieve girdle, and means for discharging said 3 145135 8/1964 Robertson et a1 162 43 fiber material from approximately the end of said vessel 3:200:032 8/1965 Richteret LL L1; 162 19 opposite said fiber material feeding means. 5 3 41 3/1966 Lang 162 237 10. The vessel as claimed in claim 9 in which a conduit and a pump are connected between said hydrolysate DONALL SYLVESTER: Pnmmy ECW711112"- and spent digestion liquor conduit and said branch conduit. FREEDMAN, Assistant Examiner. 

